1. Field of the Invention
The present invention relates to pneumatic radial tires for heavy load vehicles and more particularly to pneumatic radial tires for heavy load vehicles, which are especially suited for large-sized vehicles such as truck, bus and the like and comprise a carcass reinforced with a belt including rubberized metal cords as a tension member.
2. Description of the Prior Art
Generally, there are provided two reinforcements in the pneumatic radial tire, one of which is a carcass composed of ply cords lying in a radial plane of the tire or forming a small angle with said plane and the other of which is a belt disposed between the carcass and a tread rubber layer and including at least two rubberized cord fabrics as tension members crossing with each other at a small cord angle of not more than 25.degree. with respect to equatorial plane of the tire.
This belt exclusively bears a tensile force in the circumferential direction of the tire because the ply cords of the carcass are disposed so as to lie in a radial plane or form a very small angle with said plane. Therefore, the belt is substantially different in function from a breaker in pneumatic bias tires bearing the tensile force in circumferential direction together with a carcass ply.
Preferably, in the radial tires for heavy load vehicles such as truck, bus and the like, metal cords are employed as a reinforcement of the belt. In this case, a strong reinforcing effect is obtained in a tread portion, so that many excellent properties such as wear resistance, puncture resisting property and the like are developed.
Among such properties, however, there are still remaining various problems.
Namely, the radial tire is expensive as compared with the bias tire and hence it cannot be said that the radial tire is always dominant in view of cost though the wear resistance is a catch phrase in the radial tire.
Further, the wear resistance is relative to a tensile rigidity in the circumferential direction of the belt. Therefore, in order to enhance the wear resistance, there has been attempted to make small an array angle of metal cords with respect to equatorial plane of tire or to increase the count of metal cords used or the ply number of metal cord fabrics. However, when the array angle of metal cords in the belt is small, shear strain in both circumferential and radial directions are excessively caused in the vicinity of both edges of the belt during the rotational movement of tire under a load to bring about an interlaminar separation of the tension members, whereby the life of the tire is excessively deteriorated. Furthermore, the increase of the cord count or ply number not only causes the increase in the weight and cost, but also strongly tends to bring about the separation.
In the initial stage of the separation, cracks are caused between the metal cord and the rubber coated thereon. Then, these cracks are propagated between the adjacent metal cords so as to promote the separation. For this reason, the excessive increase of the cord count rather results in the promotion of the separation, while the increase of the ply number causes an advance of heat-build up due to inner friction, thus resulting in the development of the separation.